I. Field of the Invention
This invention relates to the field of superconducting devices. The invention involves superconductor substrates comprised of monocrystalline lanthanum orthogallate and mixed single crystals of lanthanum orthogallate. In accordance with the invention, lanthanum orthogalate, and mixed crystals thereof can be produced in the form of large perovskite-type single crystals. Specifically, the invention relates to the discovery of a new crystalline substrate on which T.sub.c superconducting thin film of metal oxide based perovskite are deposited. The substrate found suitable for this purpose is a single or mixed single crystal of lanthanum orthogallate. The crystal from which the substrate is fabricated are grown from a pure melt of lanthanum, gallium and additive oxide while controlling the major crystallographic direction of solidification.
II. Description of the Prior Art
The mineral perovskite (CTiO.sub.3) serves as a prototype crystal structure for a large class of compounds of general formula ABO.sub.3. In this structure, the A element is a large cation while the B element is a smaller cation. In order to maintain charge neutrality in the compound, the sum of the cation valence states should total six. Thus, various combinations of A and B valence states are possible, e.g. 3--3, 2-4, 1-5 and even mixed fractional compositions The ideal perovskite crystal structural is cubic where 8(A)ion are locate at the cube corners, 6(0)ions at the cube faces and a single B ion at the cube center. In a practical situation, most perovskite structures are distorted from cubic to tetragonal, rhombohederal, or orthorhombic crystal forms. The perovskite structure is likely to be generated where A cations are coordinated with 12 oxygen ions and B cations with 6 oxygen. It was shown first by V. M. Goldschmidt in Skrifter Norske Videnskops Akad, Oslo, I, Mat. - Naturv. Kl., No. 8 (1926) that a cubic perovskite is stable only if a tolerance factor has an approximate range of 0.8&lt;t&lt;0.9. Here t is defined by R.sub.A & R.sub.O =t.sqroot.2 (R.sub.B +R.sub.O where R.sub.A, R.sub.B, and R.sub.O are ionic radii of the A, B, and 0 ions respectively. For distorted structures, t may have a larger tolerance. The perovskite structure is very important as a basis for semiconductors, thermoelectrics, ferroelectrics, laser hosts, catalysts, and ferromagnetic material. A modern discussion of these applications in len in the boom "Structure, Properties, and Preparation of Perovskite - Type Compounds " by F. S. Galasso, Pergamon Pres, New York, 1969. The perovskite relate high Tc superconductor oxides of type La.sub.1.85 Ba.sub..15 CuO.sub.4-x and YBa.sub.2 Cu.sub.3 O.sub.7-x' which were discovered in 1987, have again brought this structure into technical prominence. ABO.sub.3 compounds other than CaTiO.sub.3 (perovskite) which possess the perovskite crystal structure are alternatively referred to as "perovskite-type", "perovskite-like" or "perovskite-related" A used herein when referring to compositions other than CaTiO.sub.3, the term "perovskite" describes the crystalline structure of such compositions.
The compound LaGaO.sub.3 was prepared first as a polycrytalline powder, characterized, and reported as a perovskite-like structure by S Geller, in Acta Cryst 10, 243 (1957). Geller determined by X-ray diffraction, the structure at room temperature to be an orthorhombic crystal with the following unit cell dimensions: a=5.496 A, b=5.524 A, and c=7.787 A. Small single crystals, ca, 1-5 mm in size, were grown from a PbO-B.sub.2 O.sub.3 flux as reported by M. Marezio, J. P. Remeika and P. D. Dernier in Inorganic Chemistry 7, 1337 (1968). These workers again-determined the crystals to be orthorhombic with the following lattice constants: a=5.526 A b=5.473A and c=7.767 A. Several years later a reinvestigation of the La.sub.2 O.sub.3 -Ga.sub.2 O.sub.3 system solid state reaction showed a similar result for LaGaO.sub.3. See S. Geller, P. J. Curlander and G. F. Ruse in Mat. Res. Bull. 9, 637 (1974) Geller reported measurement of the orthorhombic unit cell a follow: a=5.519 A, b=5.494 A, and c=7.770 A.
The phase diagram of the La.sub.2 O.sub.3 -Ga.sub.2 O.sub.3 system was studied in a preliminary fashion by S. J. Schneider, R. S. Roth and J. L. Waring, J. Research Natl, Bur. Standard 65A [4] 365 (1961). Schneider et al. found that the perovskite phase existed in the stoichiometric 1:1 composition. However, it is indicated whether this composition or those adjacent within few mole percent were either congruently or incongruently melting. Generally mixtures of a high melting component (La.sub.2 O.sub.3) with a partially volatile low melting component such as Ga.sub.2 O.sub.3 are difficult to control compositionally at or near the melting point of the stoichiometric mix. The prior art has not recognized that boules capable of providing monocrystalline lanthanum orthogallate or mixed single crystals of lanthanum orthogallate of predetermined orientation could be readily grown from pure melts.
Monocrytalline lanthanum orthogallate or mixed single crystals of lanthanum orthogallate have not heretofore been used as substrates for any superconductor materials.
III. Invention Contrasted From the Prior Art
In accordance with the preset invention, it has unexpectedly been found contrary to what is suggested by previous investigators that powdered stoichiometric mixtures of La and Ga at the 1:1 Ga.sub.2 O.sub.3 composition melt stably and apparently congruently with little or no evolution of Ga.sub.2 O.sub.3 ; that the resulting stable melts of about 1:1 composition are low melting, circa 1675.degree. C. by uncorrected pyrometer, a temperature near the Ga.sub.2 O.sub.3 melting point; that such pure melts can be forced and retained in iridium crucibles for sufficient time to practice the Czochralski method of growth; that such melted can be seeded by an iridium wire to obtain a single perovskite-type crystal near a preferred [110] orientation; that seed growth via oriented single crystal can produce large boule in [110] orientation or other favorable orientations, viz. [100] , [ 010], or [001]; that mixed single crystals of lanthanum orthogallate can be grown using the same method employed to produce large lanthanum orthogallate single crystals of predetermined orientation; and, that by selection and substitution of certain cations for either La or Ga cations in lanthanum orthogallate single crystals; that is, by formation of mixed single crystals of lanthanum orthogallate--a substantially lattice-matched surface for superconductor films of pervoskite-type architecture can be fabricated from single crystals or mixed single crystals of lanthanum orthogallate.